When first posted as such in January 1964, SR 520 began at Jct I-5 Exit 168B in Seattle and headed east to Lake Washington Blvd/104th Ave NE (now Bellevue Way) near the Bellevue/Houghton (now part of Kirkland) city limits. This route had opened as the Evergreen Point branch of PSH 1 in August 1963. Northup Way (incorrectly signed in places and shown on some maps as Northrup Way) was used as the temporary route of SR 520 from Lake Washington Blvd to SR 405/ I-405. By 1966, the SR 520 freeway had been extended east to I-405 and 124th Ave NE/Northup Way. In the early 1970s, SR 520 was extended further to 148th Ave NE. NE 24th St and the Bellevue-Redmond Rd were signed as Temporary SR 520 from 148th to SR 901.
A section of what was later to be SR 520 opened as SR 920 from W Lake Sammamish Pkwy NE (at the time part of SR 901) to Redmond Way/SR 202 in the mid 1970s. SR 920 was 2 lanes wide. Around 1979, the final section of SR 520 from 148th to W Lake Sammamish Pkwy opened to traffic. The Temp SR 520 signs were removed and SR 920 was reposted as SR 520. In the 1990s, the two lane SR 520 from W Lake Sammamish Pkwy to SR 202 was widened to 4 lane divided and an interchange replaced the grade level intersection at the SR 520/SR 202 junction. Today, SR 520 is 12.82 miles long. The state is studying the Evergreen Point Floating Bridge and connecting pieces of SR 520. In the future, the existing bridge may be replaced with a new, wider bridge.
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